Communication pubs.acs.org/JACS
Efficient Charge Storage in Dual-Redox Electrochemical Capacitors through Reversible Counterion-Induced Solid Complexation Brian Evanko,†,# Seung Joon Yoo,*,‡,# Sang-Eun Chun,⊥ Xingfeng Wang,∥ Xiulei Ji,∥ Shannon W. Boettcher,*,§ and Galen D. Stucky*,†,‡ †
Materials Department, University of California, Santa Barbara, California 93106, United States Department of Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States ⊥ School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea ∥ Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States § Department of Chemistry & Biochemistry, University of Oregon, Eugene, Oregon 97403, United States ‡
S Supporting Information *
ABSTRACT: The performance of redox-enhanced electrochemical capacitors (redox ECs) is substantially improved when oxidized catholyte (bromide) and reduced anolyte (viologen) are retained within the porous electrodes through reversible counterion-induced solid complexation. Investigation of the mechanism illustrates design principles and identifies pentyl viologen/bromide (PV/Br) as a new high-performance electrolyte. The symmetric PV/Br redox EC produces a specific energy of 48.5 W·h/kgdry at 0.5 A/gdry (0.44 kW/kgdry) with 99.7% Coulombic efficiency, maintains stability over 10 000 cycles, and functions identically when operated with reversed polarity.
Figure 1. Viologen/bromide redox ECs. (a) Viologen dibromide and faradaic energy storage reactions of bromide and viologen (denoted as V). (b) Cycling stability of redox ECs. Cells are charged to 1.4 V (0.5 M MVCl2/1 M KBr) and 1.2 V (0.1 M HVBr2/0.4 M KBr) at 0.5 A/ gdry in custom Swagelok cells (Figure S1).
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ommercial electric double-layer capacitors (EDLCs) provide specific power as high as 20 kW/kg for millions of cycles, but have a low energy density, compared to other energy storage systems, of ∼6 W·h/kg (8 W·h/L).1,2 To increase the energy density of EDLCs, research has focused on developing pseudocapacitive systems that add faradaic energy storage.3−10 One recent approach is to replace the traditional solid-state pseudocapacitive materials with soluble redox couples.11−16 Such redox-enhanced electrochemical capacitors (redox ECs) provide high power density and increased specific energy without the need for nanostructured metal oxide/nitride electrodes and allow for good performance in aqueous electrolytes. However, they are challenged by poor cycle lifetime and fast internal self-discharge due to cross-diffusion of redox-active electrolytes.17,18 For efficient redox ECs, the redox couples used should exhibit fast and reversible electron transfer and be retained inside the porous electrodes in their charged states (i.e., oxidized catholyte or reduced anolyte) to eliminate self-discharge.19 We recently identified heptyl viologen/bromide (HV/Br) as a promising dual-redox-active electrolyte for aqueous redox ECs (Figure 1a, R = n-heptyl).20 The system maintains cycling stability over 20 000 cycles and has a slow self-discharge rate without using a costly ion-selective membrane as a separator. However, due to the low solubility of HV (1.5 >1.5
